A variety of bill or currency validation and stacking techniques are known in the prior art, including the following U.S. Pat. Nos. 4,628,194 (METHOD AND APPARATUS FOR CURRENCY VALIDATION), 4,722,519 (STACKER APPARATUS), 4,765,607 (STACKER APPARATUS), 4,775,824 (MOTOR CONTROL FOR BANKNOTE HANDLING APPARATUS), 5,209,395 (METHOD AND APPARATUS FOR A LOCKABLE, REMOVABLE CASSETTE, FOR SECURELY STORING CURRENCY), 5,222,584 (CURRENCY VALIDATOR), 5,209,335 (SECURITY ARRANGEMENT FOR USE WITH A LOCKABLE, REMOVABLE CASSETTE), and U.S. Ser. No. 08/179,613 (CURRENCY VALIDATOR AND SECURE LOCKABLE REMOVABLE CURRENCY CASSETTE, filed on Jan. 10, 1994); U.S. Ser. No. 08/179,110 (SECURE CURRENCY CASSETTE WITH A CONTAINER WITHIN A CONTAINER CONSTRUCTION, filed on Jan. 10, 1994); and U.S. Ser. No. 08/179,113 (CURRENCY VALIDATOR AND CASSETTE TRANSPORT ALIGNMENT APPARATUS, filed on Jan. 10, 1994), all of which are assigned to the assignee of the present invention and are incorporated by reference herein.
Bill validators typically include a validation portion comprising plastic housings which include sensors for examining a bill. Light emitting diodes (LED's ), for example, are used to illuminate the bill at particular wavelengths. Phototransistors are then provided to receive the light transmitted through or reflected from the bill. The pattern of the received light can be compared to the expected pattern for an acceptable bill to determine if the bill under test is acceptable. The LED's and phototransistors can be mounted on printed circuit boards mounted to or placed within the plastic housings.
The validation portion of the bill validator is usually proximate the bill entry, near the external environment. Ambient light can therefore enter the validation portion, interfering with the reception of light by the phototransistors. One approach to minimize such interference is to make the plastic of the validation housing transparent to a particular color, such as red. Ambient light of wavelengths other than that color will be absorbed and will not be detected by the phototransistors, lessening, but not eliminating the problem. Use of a plastic transparent to only a particular color, however, limits the wavelengths which can be used to examine the bill to the color of the housing.
Opaque or black housings, which absorb essentially all visible wavelengths, provide the best suppression of ambient light. Since light cannot be transmitted through such housings, however, open portions must be provided to enable the passage of light from the LED's and to the phototransistors. Such openings allow dirt, water and air to contact the LED's and phototransistors, interfering with measurements and degrading the components.
To protect the LED's and phototransistors, clear plastic snap-in windows have been provided over the openings. Such windows, however, are not completely water and air tight, particularly when subjected to varying temperature conditions which can cause differential expansion or contraction of the plastic windows and housings. Furthermore, the windows do not always fit flush with the surrounding housing, providing an area which can collect dirt and interfere with the leading edge of the bill as it is advanced through the bill path.
Another problem confronted by bill validators is string, tape or other such devices attached to a bill. Such string can be used to remove a bill after credit has been given or a product has been dispensed. Complicated misalignment mechanisms have been proposed to prevent retrieval of the bill. See, for example, U.S. Pat. No. 4,348,656. Other techniques for preventing string fraud disclosed in the '656 patent include providing a rotating drum through which a bill passes. If a string is present, it will be rolled about the drum, preventing the retrieval of the bill through the string.
Cross-channel sensors have also been provided in validation housings to detect the presence of string or tape. A light emitting diode can be positioned on one side of the bill path and a photodetector on the other. String or tape attached to the bill can obstruct a portion of the light transmitted across the channel and detected by the photodetector. Detection of a different level of light than expected indicates that string or tape may be attached to the bill.